1. Field of the Invention
The present invention relates to polymer films. In particular, the present invention relates to high density polyethylene compositions and polymer films blown from such polyethylene compositions and having good impact and mechanical properties and processability. The invention also concerns a process for the production of the polymer compositions and the film-making process.
2. Description of Related Art
There is a growing demand for thinner films (downgauging) due to environmental aspects and cost reduction. Since film strength and performance in packaging lines should be maintained in spite of the downgauging, stiffer films with high mechanical properties are needed to meet future demands.
The polyolefin materials used for blown film applications can be divided into the following five groups of materials:
1) Low Density Polyethylene (LDPE) having broad molecular weight distribution (MWD) and produced by radical polymerisation in one reactor;
2) Linear Low Density Polyethylene (LLDPE) having narrow MWD and produced by polymerization in the presence of Ziegler catalysts in one reactor
3) High Density/Medium Density Polyethylene (HD/MDPE) having broad MWD and produced by polymerization in the presence of Cr catalysts in one reactor
4) High Density Polyethylene (HDPE) having a bimodal (broad) MWD) and produced by polymerization in the presence of Ziegler catalysts in two reactors in series (cascade)
5) Linear Low Density Polyethylene (LLDPE) having a bimodal (broad) MWD and produced by polymerization in the presence of Ziegler catalysts in two cascaded reactors.
The market for PE films is continuously developing improved solutions to packaging requests based on the above type of materials.
Unimodal HD film materials (point 3 above) have a high stiffniess, but limited processability and mechanical properties. The density may be reduced to improve the mechanical properties but this results in reduced stiffniess.
Bimodal HD film materials (point 4 above) also have high stiffness, but compared to unimodal materials they have good processability and good mechanical properties over a wider range of processing conditions as well. Typically these materials have been produced in cascaded slurry reactors. Said processes are suitable for production of HDPE materials, but unfortunately they are not flexible enough to allow for the production of MDPE and LLDPE materials.
It is also possible to produce a bimodal HDPE film material in a more flexible process, like the one presented in EP 517 868 B1, comprising a combination of slurry and gas phase reactors, which provides for a wider variation of polymerization conditions. However, attempts to reproduce the conventional materials made in a cascaded slurry process in this kind of a process have usually resulted in a material which cannot be used in film applications, due to either a limited processability or a too high gel level. Similar problems are encountered in processes comprising cascaded gas phase reactors.
It is an object of the present invention to eliminate the problems of the prior art and to provide novel blown polymer films. In particular the invention aims at eliminating the problems of the production of HDPE materials in multireactor processes comprising combinations of slurry and gas phase reactors or cascaded gas phase reactors.
It is another object of the present invention to provide a process for producing suitable polyethylene materials for production of films, in particular blown films.
These and other objects, together with the advantages thereof over known processes and products, which shall become apparent from, the specification which follows, are accomplished by the invention as hereinafter described and claimed.
As described above, the currently used HD film materials have generally been produced in cascaded slurry reactors. It has now been found that materials which can be used for making high density PE films with good processability and good properties can also be produced in a process comprising a cascade of loop and gas phase reactors or several gas phase reactors.
The invention is based on the finding that the properties of HD film materials made in slurry processes can be matched by a special combination of the average molecular weight. and molecular weight distribution which is produced in the above-mentioned processes. Surprisingly, the properties of the films made from such materials will be equal or even improved compared to films produced from conventional bimodal HDPE.
The specific combination of average molecular weight and molecular weight distribution is manifested in a reduced shear thinning index and increased storage modulus in comparison to conventional materials. The shear thinning index of the materials according to the present invention can be defined as follows:
SHI5/300xe2x89xa60,00014xcex75kPa+78
wherein
xcex75kPa is complex viscosity at G*=5 kPa and
SHI5/300 is the ratio of complex viscosity at G*=5 kPa to the complex viscosity at G*=300 kPa.
The present material is preferably produced by polymerizing or copolymerizing ethylene in a reactor cascade formed by at least two reactors, one of which is a gas phase reactor, operated with different amounts of hydrogen and comonomers to produce a high molecular weight portion in one of the reactors and a low molecular weight portion in another so as to provide a bimodal high density polyethylene with a low molecular weight part having a density above 960 kg/m3 and a high molecular weight part, the composition having a density of 940-965 kg/m3 and MFR21 of 3-50.
More specifically, the present polyethylene composition is characterized by what is stated in the characterizing part of claim 1.
The process according to the present invention is mainly characterized by what is stated in the characterizing part of claim 7.
The present process for producing high density polyethylene films is characterized by what is stated in the characterizing part of claim 15.
The present polyethylene film is characterized by what is stated in the characterizing part of claim 21.
The advantage of the present invention is that it provides a material for making high density PE blown films with good mechanical properties and good appearance in a process where the whole range of PE products from LLD to HD can be produced. The material is comparable to that produced in cascaded slurry reactors, allowing the production of HD products only. Further, the use of a light diluent in the slurry reactor and at least one gas. phase reactor reduces the amount of volatiles present in the product. Such volatiles typically stem from heavy diluents used in slurry polymerization. Typically, the amount of volatiles in the HDPE material according to the present invention is less than 500 ppm, in particular less than 300 ppm.
The tear strength, impact strength and processability on a film line make the present materials useful for production of thin films of thicknesses in the range of 5 xcexcm, or even less than 5 xcexcm, to over 30 xcexcm. Films made from the materials also exhibit good barrier properties to water vapor.
Next, the invention will be more closely examined with the aid of the following detailed description and with reference to the attached drawings.